how can i vectorize this loop?

% x is a vector given in main program

function y = userfunction2(x,k)

x=y;

for i=i:k

y=userfunction(y);

end

[Akira Agata]

I don't think this function works.

What is the purpose of the second line ('x=y;')? Why input variable x is replaced by y? What is y??

What is the userfunction(y) in the third line?

[William Brannon]

function y = LeftShift(x)

% x is a row n-vector and y is a row n-vector that is the left shift of x.

n=length(x);

y=zeros(n,1);

for k=1:n

      if k<n;
          y(k)=x(k+1);
      else
          y(n)=x(1);
      end
  end

[William Brannon]

It is a fuction to shift a given vector. the first value in hte vector goes to the nth place, the other values shift 1 position to the right.

[William Brannon]

the original function works, but I am supposed to do it without a loop. LeftShift is okay with a loop,but the other one is supposed to shift it k times, which is entered by another user.

[William Brannon]

What I really want is afunction that does this without a loop "k" times:

ShiftLeft(...(ShiftLeft(y)...)

Answer 1

[Walter Roberson]

See fold()

[William Brannon]

I'm sorry, but I don't understand how this works. What would the code actually look like?

I was thinking of vector operations to complete it, but I am not that familiar with how to use it in this case.

[Walter Roberson]

ShiftLeftAux = @(V) {ShiftLeft(V)}
t = cell(1,k); t{1} = {y});
t2 = fold(ShiftLeftAux, t);
result = t2{1};

Not nearly as clean as I would like.

This does not really vectorize the loop, though: it merely hides the loop into fold(), the same as if you had just written a routine that did the operation multiple times.

If you want actual vectorization you should instead be using circshift().

There is no way in MATLAB to say "vectorize this arbitrary linear code": there are only ways to get it to apply the linear code without you writing a visible for loop. The results are typically slower than using a for loop.

Answer 2

[Akira Agata]

Seems that you would like to do circshift ?

Your 'y = ShiftLeft(x)' seems equivalent to 'y = circshift(x,1)', like:

x = 1:10;
y = circshift(x,1);

Then, y becomes

>> y
ans =
  10     1     2     3     4     5     6     7     8     9

If you want to shift k times, you should simply do 'y = circshift(x,k)'. Please see more detail on circshift documentation page .